In recent years, the advancement of liquid crystal display elements is remarkable, and most of these display elements are concerned with the application of nematic liquid crystals. On the other hand, other liquid crystal displays of different operation modes, in which characteristics of the other liquid crystal phase are utilized, have been intensively researched with the intention of putting these liquid crystal displays into practice, and as one kind of these other liquid crystal displays, there is a thermal writing projection device utilizing a thermal optical effect of a smectic A phase. The system of such a thermal writing projection device can provide the display of a larger area and a higher information capacity, both in the order, as compared with a conventional TN mode (twisted nematic mode).
Characteristics required for the liquid crystal materials used in this field are as follows:
(1) A wide smectic A phase range should be present (preferably, the smectic A phase should exist even at a temperature of 0 to -20.degree. C. or less). PA1 (2) A nematic phase range should be extremely narrow. PA1 (3) Birefringence should be large. PA1 (4) Specific heat and phase transition energy should be small. PA1 (5) Dielectric anisotropy value should be large. PA1 mp: Melting point (.degree.C.) PA1 SN: Smectic/nematic transition point (.degree.C.) PA1 NI: Nematic/isotropic transition point (.degree.C.) PA1 NR: Nematic range (.degree.C.) (=NI (.degree.C.) - SN (.degree.C.)]
Of these requirements, the paragaraphs (1), (2) and (3) are considered to be important for the sake of the improvement of display performance such as the contrast on a projection screen. The liquid crystal material having the above-mentioned characteristics has not been found in the form of a single compound at present, and hence each mixture of various liquid crystal compounds is now investigated. Examples of these liquid crystal compounds can be represented by the following general formulae: ##STR3##
Furthermore, exemplary compositions of these compounds are set forth in Table 1.
TABLE 1 ______________________________________ mp SN NI NR Composition Compounds (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) ______________________________________ a I.sub.8, I.sub.10, I.sub.15 24 73.5 74 0.5 b II.sub.8, II.sub.10 12 37 42 5 c II.sub.8, II.sub.10, V.sub.8 6 53 56 3 d II.sub.8, II.sub.10, IV.sub.9 8 41.5 47 5.5 e II.sub.8, III.sub.10, IV.sub.9 -30 46.8 51.0 3.2 f II.sub.8, II.sub.10, III.sub.10 -32.7 54.7 57.3 2.6 III.sub.12, IV.sub.9, VI.sub.8 ______________________________________
In Table 1, the respective abbreviations have the following meanings:
In the column of "Compound" in the table, for example, I.sub.8 means the compound of n=8 of Formula (I) mentioned above.
With regard to the composition a, the nematic range is as extremely narrow as 0.5.degree. C., but the melting point is high, 24.degree. C., which is in the vicinity of room temperature. Therefore, this compound is not practical, though it seems to be usable in experiments. On the other hand, with regard to the compositions e and f, the melting points are low, -30.degree. C. and -32.7.degree. C., respectively, but the nematic ranges are wide, 3.2.degree. C. and 2.6.degree. C., respectively. In consequence, these compositions are not satisfactory.
The nematic range is a factor which has an influence on the writing velocity and the display contrast of the display elements, and when the nematic range is wide, the writing velocity is low and the contrast also is poor. That is, when thermal writing is carried out under conditions of the wide nematic range, a long cooling time is required in the nematic phase in the step of forming an opaque smectic phase from an isotropic liquid via a nematic phase, which causes the writing velocity to delay. On the other hand, when the nematic phase state remains for a long period of time, the liquid crystal is oriented by a substrate which has been subjected to an orientation treatment (vertical orientation or homogeneous orientation), so that the scattered smectic A phase cannot be obtained, which causes the contrast to deteriorate.
With regard to the melting point, if it is 0.degree. C. or less, the composition having such a melting point can be used in its usual applications. However, when it is intended to introduce long-term memory properties into a liquid crystal element, the melting point of 0.degree. C. or so is insufficient and thus a lower melting point is demanded.
In addition to the above-mentioned compositions, the other thermal writing liquid crystal compositions are disclosed in Japanese Patent Laid-open Publication Nos. 144383/1985, 252686/1985 and 268790/1986, but the compositions having a nematic range of 2.degree. C. or less and a melting point of 0.degree. C. or less simultaneously cannot be found anywhere.
An object of the present invention is to provide a liquid crystal composition for thermal writing liquid crystal elements, more specifically, a liquid crystal composition having an extremely narrow nematic range and a low-melting phase transition point, and more particularly the present invention is to provide liquid crystal elements using the aforesaid composition.